The Sig fabric will hold up well under wet conditions. I got caught in some pretty bad rain and most of the water just beaded up and rolled off my bag. Once I got inside, it felt a teensy bit damp, but it didn't soak through or anything...and the bag still looks brand new (yes, it was the chocolate sig too!) 
Any signature I've had has held up well. I have the gallery tote in khaki and its been in the rain a couple of times and looks great. I don't deliberately try to keep out longer than I have to but at times you can't help it! I do have fabric cleaner but haven't used it on the gallery tote yet.
I have a chocolate siggie bag (bleecker) that I use in bad weather and love it! I worry about my leather bags in rain too much to carry them so use the signature! And brown signature is gorgeous!